A magnetic sensor is a device that measures the strength of an external magnetic field. There are a number of different approaches to measuring magnetic fields, and various different types of magnet sensors have been developed based on these different approaches. One type of magnetic sensor based on flux variations in a magnetic core is a fluxgate sensor.
FIG. 1 shows a block diagram that illustrates an example of a prior art fluxgate sensor 100. As shown in FIG. 1, fluxgate sensor 100 includes a drive coil 110, a sense coil 112, and a magnetic core structure 114 that lies within drive coil 110 and sense coil 112.
FIGS. 1A-1B show views that illustrate the operation of fluxgate magnetic sensor 100. FIG. 1A shows a graph that illustrates a BH curve 200 for magnetic core structure 114, while FIG. 1B shows a graph that illustrates the magnetic induction field B that results from the alternating current input to drive coil 110, FIG. 1B also shows a graph that illustrates an induced voltage in sense coil 112 plotted in the time domain that results from the magnetic induction field B, and FIG. 1C shows a graph that illustrates the induced voltage in sense coil 112 plotted in the frequency domain that results from the magnetic induction field B.
As shown by BH curve 200 in FIG. 1A, when the magnitude of a magnetic field H increases, magnetic core structure 114 increases the magnitude of the magnetic induction field B until magnetic core structure 114 saturates. Once in saturation, further increases in the magnitude of the magnetic field H lead to very little increase in the magnitude of the magnetic induction field B. As a result, saturation is commonly illustrated as in FIG. 1A as the region where increases in the magnitude of the magnetic field H lead to no additional increase in the magnitude of the magnetic induction field B.
In the present example, the magnitude of the magnetic field H is increased by increasing the magnitude of the alternating current flowing through drive coil 110.
As shown in FIGS. 1A-1B, when no external magnetic field is present and an alternating current waveform, which has an amplitude that is sufficient to drive magnetic core structure 114 into saturation, is input to the drive coil 110, an alternating magnetic induction field B, is generated in response. In other words, when alternating current waveform is input to drive coil 110, magnetic core structure 114 is driven through cycles (magnetized, un-magnetized, inversely magnetized, un-magnetized, magnetized again, and so on) that generate an alternating magnetic induction field B. In the present example, the alternating current waveform on coil 110 is triangular, while the magnetic induction waveform on coil 112 has flat tops and bottoms that represent the periods of saturation.
As a consequence the alternating magnetic induction field B induces an alternating voltage in sense coil 112. The induced alternating voltage is proportional to the change in the magnetic induction field B over time (dB/dt).
As shown in FIG. 1C, in the frequency domain, the induced alternating voltage has a fundamental frequency 1 f, but only odd harmonics, such as a third harmonic 3 f, of the fundamental frequency 1 f.
As a result, when no external magnetic field is present, the induced alternating voltage has no second harmonic. However, when an external magnetic field is present, the external magnetic field interacts with magnetic core structure 114 and changes the alternating magnetic induction field B. In other words, magnetic core structure 114 is more easily saturated when magnetic core structure 114 is in alignment with the external magnetic field, and less easily saturated when magnetic core structure 114 is in opposition to the external magnetic field.
Magnetic films used in the fluxgate devices have the directional dependence at the operation frequency of interest (MHz range). In general the direction corresponding to the hard axis of the magnetic core is utilized to measure the magnetic field. This is fine for measuring the magnetic field in one specific direction, but this makes it difficult to measure the in-plane magnetic field, which is essential for e-compass applications. This is due to the different mechanisms involved for the magnetization process along the easy- and hard-axis directions (i.e. two orthogonal in-plane directions in magnetic films). The main mechanism along the easy-axis is the magnetic domain displacement, where that along the hard-axis is the spin rotation. While the magnetization response along the hard-axis is stable over a wide range of frequency, that along the easy-axis shows a strong frequency dependence with the decreased response at MHz range. So the hard-axis direction is being utilized for the fluxgate applications. Thus, there is a need for a smaller and less expensive fluxgate magnetic sensor. A method needs to be developed that minimizes the directional dependence of magnetic sensor devices.